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Sima de los Huesos hominins are proto-Neanderthals

A new study, published in the journal Science, has provided support for the ‘accretion’ model of Neanderthal evolution. ‘Classic’ Neanderthals, i.e. humans possessing the full suite of Neanderthal characteristics, do not appear in the fossil record until 130,000 years ago. However, French palaeoanthropologist Jean-Jacques Hublin has proposed that Neanderthal characteristics appeared gradually over time, in a piecemeal fashion.

Thus, for example if Feature X appeared in one population and Feature Y in another, then interbreeding between the two populations would have resulted in a population possessing both Features X and Y. Over time, populations gradually acquired the full suite of Neanderthal characteristics by a process of accretion, resulting in a gradual transition from Homo heidelbergensis to Neanderthal. The accretion model explains ‘proto-Neanderthal’ features seen on certain fossils dating to the period prior to the appearance of the ‘classic’ Neanderthals. These include a 400,000-year-old fragmentary skull from Swanscombe in England and the 225,000-year-old Steinheim skull from Stuttgart, Germany.

Much of the evidence we have regarding Neanderthal origins comes from a single site in the Sierra de Atapuerca of northern Spain, near the city of Burgos: a Middle Pleistocene human burial pit known as Sima de los Huesos. The name translates – rather appropriately – as ‘the Pit of Bones’. Sima de los Huesos is a small muddy chamber lying at the bottom of a 13 m (43 ft.) chimney, lying deep within the Cueva Mayor system of caves. Investigation of the site has proved to be long and difficult. The most immediate problems are logistical. The cramped site is located more than 500 m (⅓ mile) from the mouth of the Cueva Mayor and is hard to access, necessitating at times crawling on the stomach. Another problem is the disturbance to the site caused by the many generations of souvenir and fossil hunters. Systematic excavation commenced in 1984 and has continued ever since. To date, over 2,000 fragmentary hominin fossils have been recovered, including three skulls. In total, the remains are thought to represent at least 32 individuals of both sexes. It is likely that the site was simply used for the hygienic disposal of the dead, because there is no evidence to suppose that any of the individuals were deliberately killed and the bones show no sign of injuries caused by spears or clubs.

Study of this enormous collection of bones is still in progress, and is likely to continue for some time yet as the site yields further fossils. However, it has become clear that the fossils show a mixture of Homo heidelbergensis and Neanderthal characteristics, just as would be expected if the accretion model is correct. The key question is how old is the site? Uranium series dates obtained in 2007 suggested that they were at least 530,000 years old, making the Sima people older than some Homo heidelbergensis remains from southern Europe and the Balkan region that show no incipient Neanderthal characteristic features.

The new study considered 17 crania, including seven new specimens. The sample shows a consistent morphological pattern with derived Neanderthal features present in the face and anterior of the cranial vault, many of which are adaptations to aid chewing of food. This suggests that facial modification was the first step in the evolution of the Neanderthal lineage, consistent with the accretion model evolution, with different anatomical features evolving at different rates.

The researchers also used a variety of techniques including combined electron spin resonance/uranium series to obtain a revised date of 430,000 years old, which gives a far better fit with the accretion model.